no-lines: avoid leaving an empty line instead of the syncline

Currently, with --no-lines, instead of "#line file line\n", we emit
"\n".  Let's emit nothing.

* data/skeletons/bison.m4 (b4_syncline): Emit at end-of-line when enabled.
* data/skeletons/bison.m4, data/skeletons/c.m4, data/skeletons/glr.cc,
* data/skeletons/lalr1.cc, src/output.c: Use dnl after b4_syncline to
avoid spurious empty lines.

* tests/synclines.at (Sync Lines): Make sure that --no-lines is like
grep -v #line.
* tests/calc.at: Make sure that a rich grammar file behaves properly
with %no-lines.

data/skeletons/glr.c

@@ -202,8 +202,8 @@ b4_copyright([Skeleton interface for Bison GLR parsers in C],
 ]b4_cpp_guard_open([b4_spec_header_file])[
 ]b4_shared_declarations[
 ]b4_cpp_guard_close([b4_spec_header_file])[
-]b4_output_end
-])])
+]b4_output_end[
+]])])
 
 
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